In a radio frequency (RF) receiver which uses a balanced mixer or an image rejection mixer to provide the IF signal, it is often required to provide transform phase relation ships of RF inputs, LO inputs, or IF outputs. By using the balanced mixer or the image rejection mixer the effect of even order harmonics of the L.O. and RF input are substantially reduced. Therefore, significant improvement in noise performance as well as intermodulation (IM) distortion of the receiver is realized.
The mixer is generally integrated within an IC substrate using well-known integrated circuit technologies. The IC substrate may be a GaAs, silicon or similar appropriate substrates upon which the balanced mixer is disposed.
A transformation network is required to transform a single input to differential (i.e., 180 degree out of phase) or quadrature (i.e., 90 degree out of phase) outputs which drive the inputs of the mixer. Also needed is another transformation network to transform outputs of the mixer to a single output. Generally, such transformation networks are bidirectional and the same structure may be used to perform both transformation functions.
In view of the constant drive to reduce the size and cost of the receivers, it is desired to integrate the transformation networks and the mixer on the same substrate. In this way, small and easily manufacturable receivers may be obtained without degrading the specification improvements provided by using balanced or image rejection mixers.
Conventionally, coil transformers are widely used as transformation networks in a variety of applications and provide wideband conversion from a single-ended port to balanced ports and vice versa. However, coil transformers are expensive and bulky, in addition to complicating the manufacturing process. Most importantly, they cannot be integrated on the same IC substrate as that of the balanced mixer.
The transformation networks may also be produced using transmission lines or discrete components. However, these components are not integratable. Additionally, at low frequency applications, the size of these components becomes substantially large, which further limits their use in highly miniaturized receivers.
Another method for providing a single input to a balanced output, comprises using differential amplifier pairs. This circuit, though integrated, consumes current, produces distortion, and has an inherent phase and amplitude error in the output.
It is, therefore, desired to provide a transformation network which can be integrated with a balanced mixer circuit on a single substrate to produce a small sized, transceiver.